1. Field of the Invention
This invention relates generally to the field of automotive engine diagnostics and more particularly concerns the problem of finding one or more intermittently faulty engine cylinders.
2. Description of the Prior Art
It is well known that the instantaneous angular velocity of an internal combustion engine at a fixed throttle setting varies over a small range as each cylinder fires. For an engine with all good cylinders, this variation resembles a sinusoidal function superimposed on a constant angular velocity. The pattern changes when one or more cylinders is not supplying its full power contribution. Even though this waveform is indicative of the overall performance of an engine; alone, it is not very useful for determining which cylinder is faulty, especially when there is more than one bad cylinder.
Prior art methods have made various attempts to use this angular velocity waveform to determine the condition of a cylinder. Some methods arrive at an average or statistical evaluation of a given cylinder based on low resolution angular velocity estimates averaged over many engine cycles to lead to conclusions about each cylinder's performance. There are two major disadvantages with these methods. 1) Transient phenomena such as single mis-fires or conditions that occur only rarely cannot be detected. 2) The engine cannot be tested during acceleration or changing load where the most demands are placed on it for performance.
Other methods have computed acceleration or have compared relative power against stored parameters for good engines. Acceleration methods suffer from numerical noise problems because estimating a mathematical derivative is a numerically noisy process. Comparing parameters against those of good engines requires large data bases of data for different engine types.
Schroeder and Ryan in U.S. Pat. No. 4,539,841 discuss determining peak cylinder pressure and relative power output of each cylinder from a good engine at idle speed, as well as a total load torque and total engine inertia. These good engine parameters are stored and used for comparison during diagnostics. A diagnostic uses the difference in the squares of the instantaneous RPM values at three points during the cylinder firing cycle to derive a cylinder net power output. These numbers are displayed on a CRT screen for human interpretation. The method is cumbersome in that it requires pre-stored values of several parameters such as engine inertia and engine frictional torque from a good engine in order to derive net power output. The method presented by Schroeder and Ryan is primarily suited to a four cylinder engine. The technique becomes more and more difficult to use as the number of cylinders increases because of the overlap between compression and power phases of various cylinders.
Schroeder et at. in U.S. Pat. No. 5,132,909 address a method where a very low resolution velocity estimate of six samples per engine revolution are used. Only gross statistical averages of engine speeds can be obtained. These estimates are used to derive average quantities called relative compression and relative power output for a given cylinder. While this method can possibly locate continuous or long term problems, it will entirely miss any cylinder condition that occurs intermittently or infrequently.
Obayashi et at. in U.S. Pat. No. 4,691,286 teach a closed-loop method that also uses a very low resolution angular velocity estimate (twelve samples per revolution). While different from that of Schroeder, it is still highly statistical in nature and yields only average quantities. It consists of taking the difference in the squares of velocity at two different points in the engine cycle. This quantity is then statistically analyzed over one hundred engine cycles to determine the standard deviation of performance. The standard deviation is then used in a closed loop to control the fuel-air ratio or other operating parameters. The method cannot detect transient problems or problems that only occur under certain engine loading or acceleration conditions. It is aimed at the performance and control of the entire engine, not the diagnosis of one cylinder.
Rizzoni and Connolly in their paper, "Real Time Estimation of Engine Torque for the Detection of Engine Misfires," presented at the 1991 ASME Winter Meeting, Atlanta Ga., teach a method of deconvolution from high resolution angular velocity data to torque values for each cylinder in the spatial frequency domain. Computations are performed on the 1/N th harmonic of the major rotational spatial frequency. An estimate is made of the root-mean-square of the indicated torque output of an individual cylinder. Identification of mis-fire is accomplished using statistical detection methods such as a likelihood ratio test on a non-uniformity metric. Again, the method is statistical and would miss a single event. Also, it is very compute intensive since it uses the Fast Fourier Transform and several statistical detection integrals. This makes it difficult to incorporate into a piece of standard diagnostic test equipment.
The present invention overcomes the problems caused by statistical computation and low resolution sampling, as well as computation of acceleration and comparison against stored parameters of good engines. Two numerical quality parameters are computed for each cylinder during every firing cycle. The sequence of these numbers can be stored and used to detect single mis-fires and other transient events. The method is equally applicable to engines with from two to twelve cylinders.